1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type has hitherto been known in which the driving force of a motor is transmitted through a speed-reduction gear to a steering shaft by means of gears or belts so as to assist the steering operation of the steering wheel exerted by an operator. In such a conventional power steering system, the motor is usually disposed inside the passenger compartment due to lay-out restraints, or when it is disposed in an appropriate place outside the passenger compartment such as, for example, in the engine compartment, the motor is operatively connected with the steering shaft through a flexible wire, a universal joint or the like.
The conventional power steering system as constructed above is disadvantageous in that when the motor is disposed inside the passenger compartment, the disturbing operating noises of the motor are directly transmitted to the operator. Also, in cases where the motor is disposed outside the passenger compartment and connected to a steering shaft through a flexible wire, a great deal of noise will generally be produced. Further, when using universal joints, there will be no noise generated but instead there will be irregularities in the torque transmitted so that the assistance afforded to a steering shaft by the motor will become irregular, thus impairing smooth power-steering operation. Moreover, due to time lags in response of the speed-reduction gear and/or the power transmission mechanism, the power steering system is liable to be caused to vibrate, generating irregularities in steering torque and/or impulses. Furthermore, at the beginning of a fast steering operation, or when countersteering, there will be drag or heavy steering feeling due to time lags in the rise of the operation of the motor. As a result, smooth steering is further impaired.
On the other hand, in another type of power steering system, a further or auxiliary pinion shaft is provided separately from the usual pinion shaft connected with a steering wheel. This auxiliary pinion shaft is operatively connected with a motor for driving a racked steering shaft engaging therewith so that the auxiliary pinion shaft is driven to rotate by means of the motor through the intermediary of a speed-reduction gear. In this power steering system, however, it is necessary to control motor torque with respect to the rotational force of the steering wheel wherein radio wave noises will be produced during heavy current chopper contro. Moreover, in cases where a vehicle is travelling at more than intermediate speeds and can be steered lightly without any assistance from the motor, or where the motor fails to rotate properly due to circuit trouble, or where the steering speed is increased to a value greater than the rotational speed or rpm of the motor under no load during a fast steering operation, the motor with a speed-reduction gear will become a load having a speed-increasing mechanism when the steering shaft is to be rotated by the steering wheel so that the inertia of the motor armature impairs followability of the steering system as a whole, thus requiring greater steering torque.